During formation of a well, a hole is drilled in the earth and a metal casing is inserted into the hole. After the metal casing is in place, cement is used to fill in the area between the metal casing and the wall of the hole. It is important for the integrity of the well that the cement properly cure, create a solid bond between the outside of the metal casing and the wall of the hole, and be free of voids and/or air pockets—together known as cement bond quality. If there is a problem with cement bond quality, the integrity of the well may be compromised and the problem must be fixed.
Moreover, during the life of the well, problems such as deterioration of the cement bond quality as well as with the metal casing may cause the integrity of the well to be compromised. It is therefore crucial to periodically determine well integrity by taking non-destructive measurements within the well for determination of well integrity.
As explained above, after the well hole is bored a metal casing or tube is placed into the well hole. The deeper the well the thicker the metal casing needs to be. Thickness of the metal casing thus depends upon its depth. Metal casing thickness can range from one quarter inch (¼″) to one and one quarter inches (1¼″) for shallow to deep wells.
While current technology provides various manners of non-destructive determining of well integrity, pressure and temperature considerations, particularly in deep wells (e.g. −180° C. and 20,000 psi), limits the type of technology utilized both functionally and economically. Therefore, non-destructive acoustic technology is preferred for well integrity determination. Acoustic technology utilizes an acoustic signal that is emitted outwardly from an acoustic transmitter or transducer (element) toward the metal casing, the acoustic element typically a piezoelectric element. An acoustic receiver or the transducer, again typically a piezoelectric element, receives acoustic signals that reflect from the metal casing, the cement and/or area behind the metal casing, and the well wall. The received acoustic signals are then analyzed to determine metal casing integrity and cement bond quality, i.e. well integrity.
In order to obtain meaningful data for well integrity determination, it is important that the emitted acoustic signal be at the resonant frequency of the metal casing. For metal casings of between one quarter inch (¼″) and one and one quarter inches (1¼″) thick, the resonant frequency is between 250 kHz and 500 kHz (see FIG. 9). Because a single piezoelectric transducer has a small bandwidth, it is necessary to have multiple piezoelectric transducers so as to cover the large 250 kHz to 500 kHz bandwidth. Therefore, in order to try and cover this large bandwidth, well integrity tools currently have multiple, single-element piezoelectric transducers each of a different frequency. However, because of space and cost considerations of well tools the number of piezoelectric transducers is limited. As such, current well tools do not have the capability to cover the large bandwidth necessary to perform well integrity analysis on a sizable range of metal casing thicknesses.
What is therefore needed is a compact single acoustic device that covers a large bandwidth particularly, but not necessarily for well integrity measurements.